1. Field of the Invention
The present invention relates to a process for separating sulphur in the form of hydrogen sulphide from clarified green liquor obtained from the combustion of waste liquor, a process in which the green liquor is precarbonated with a gas containing carbon dioxide and is then fed, together with a bicarbonate addition, to the separation of hydrogen sulphide, or stripping, whereby an alkali carbonate is simultaneously produced, and the bicarbonate requisite in the process is prepared by carbonation from an alkali carbonate solution.
2. Description of the Prior Art
Several processes are known for the separation of sulphur in the form of hydrogen sulphide from an alkali sulphide solution or from an alkali sulphide solution containing alkali carbonate.
Some of these are the Sivola process, the Mead process, the Stora process, and the Tampella process.
In the Tampella process the sulphide of the alkali carbonate and the alkali sulphide solution first reacts at the precarbonation stage with the carbon dioxide of the flue gases, thereby forming alkali bisulphide and alkali carbonate. The separation of sulphur is based on a chemical reaction in the liquid phase between the alkali bisulphide and the alkali bicarbonate, whereby hydrogen sulphide and alkali carbonate are produced. The separation of the hydrogen sulphide is preferably carried out in a multiple bottom column by steam stripping according to Finnish Patent No. 45,576. The method used can be direct steam stripping, or a calorisator can be used, whereby even alkali carbonate can, when so desired, be crystallized for the preparation of the digesting solution according to U.S. Pat. No. 3,508,863.
In the Tampella process the alkali bicarbonate required for the separation of hydrogen sulphide is prepared from alkali carbonate in a separate carbonator. This alkali carbonate has usually been crystallized in the lower part of the stripping unit, or a so-called make-up alkali carbonate is used. In the carbonation reaction, carbon dioxide from the flue gases is absorbed into the alkali carbonate, whereby alkali bicarbonate is formed. The requisite flue gas is often taken from the furnace for burning waste liquor, CO.sub.2 14-16 %, or even more advantageously from a lime sludge furnace, where the carbon dioxide content in the dry gases is 20-22 %. The gases are cooled with water at a separate cooling stage from 120.degree.-160.degree. C. to 35.degree. C. A great amount of gas is used in the carbonator because the gases pass through the apparatus only once, whereby the utilization degree of carbon dioxide remains under 10 % of the total carbon dioxide amount. The alkali carbonate used for the carbonation must be devoid of sulphide in order that a non-desirable generation of hydrogen sulphide be prevented at this stage, for it would pass into the environment along with the flue gases.
The utilization degree of carbon dioxide in the Tampella process is 15-20 %, when including the precarbonation and carbonation in the calculation.